FIG. 1 illustrates a cross sectional view of a selected stage in the prior art fabrication process of MTJ MRAM arrays after MTJ etching. A memory element of MRAM typically includes of a bottom electrode, a MTJ (Magnetic Tunnel Junction) and a top electrode (TE) 6. The top electrode layer 6 can be a single layer metal or multi-layer stack consisting of metal and dielectric materials. The MTJ is formed with a barrier layer 4 such as MgO sandwiched between a top magnetic layer 5 and a bottom magnetic layer 3. At the stage shown in FIG. 1 the MTJ layers have been etched, but the bottom electrode layer 2 has not been etched. The bottom electrode layer 2 has been deposited over the typical CMOS control structures 1.
Magnetoresistive random access memory (MRAM) cells including magnetic tunnel junctions (MTJ) memory elements can be designed for in-plane or perpendicular magnetization of the MTJ layer structure with respect to the film surface. One of the magnetic layers is designed to serve as a free magnetic layer while the other one has a fixed magnetization direction. The resistivity of the whole MTJ layer stack changes when the magnetization of the free layer changes direction relative to that of the fixed layer, exhibiting a low resistance state when the magnetization orientation of the two ferromagnetic layers is substantially parallel and a high resistance when they are anti-parallel. Therefore, the cells have two stable states that allow the cells to serve as non-volatile memory elements.
The MRAM cells in an array on a chip are connected by metal word and bit lines. Each memory cell is connected to a word line and a bit line. The word lines connect rows of cells, and bit lines connect columns of cells. Typically CMOS structures 1 include a selection transistor which is electrically connected to the MTJ stack through the top or bottom metal contacts. The direction of the current flow is between top and bottom metal electrodes.
FIG. 1 shows a selected stage in the fabrication process after MTJ etching using a conventional mask etching process steps such as lithography and reactive ion etching (RIE). MTJ etching chemistry may create surface damage 7 on sidewall with a depth δ. It should be removed in the next step. The removal process is strongly dependent on the sidewall angle α.
Ion Beam Etching (IBE) has been widely used in various industries for patterning thin films. It is convenient to etch hard materials with chemical etching processes such as RIE (Reactive Ion Etch). It is, however, difficult to find a hard mask material with enough selectivity for use with RIE. Re-deposition of etched material on the sidewall is also a serious concern, because it can make it the device inoperable by forming an electrical short across the barrier layer.
FIG. 2 illustrates a cross sectional view of a selected stage in the prior art fabrication process of bottom electrode etching with normal incidence. A conventional bottom electrode etching process often uses chemistry free etching using Ar, Kr and so on in which the etching products are not volatile. Re-deposition from the top electrode and/or the bottom electrode is a serious concern as illustrated in FIG. 2. When a conductive material is re-deposited on the MTJ sidewall at the barrier layer, the top and bottom magnetic layers are shorted. The re-deposition depends on sidewall slope.
Etching and re-deposition occur simultaneously. When the deposition rate is larger than the etching rate, re-deposition material accumulates on the sidewall. When the etching rate is higher, the sidewall is cleaned up. In vertical directional etching with etch rate ER, the lateral etch component is estimated by (ER/Tan α), where α is the slope of the sidewall. Shallow slope is helpful for preventing the re-deposition. However, it is not desirable for controlling the MTJ size and its uniformity for scalability. This vertical etching method removes top electrode thickness by (δ×Tan α) in order to remove thickness δ of the damaged sidewall layer 7. This amount of top electrode thickness loss is not desirable and would make downstream interconnect process difficult.
The higher or more vertical the slope, the more susceptible it is to re-deposition since the lateral component of etch rate in directional etching ambient such as IBE is, in general, less than the vertical component. Using a tilted incident ion beam increases the etching rate of re-reposition material and thus reduces the net re-deposition. It is not desirable to expose MTJ sidewall to atmosphere for wet cleaning. IBE can advantageously clean the sidewall without exposing to atmosphere.
However, IBE is a purely physical etching process, so the etch rate does not vary greatly among various materials. In other words, IBE material selectivity is low. Specifically, IBE etching selectivity of a hard mask layer versus magnetic materials is not as desirable as that of a chemical etch process such as RIE. A very thick hard mask is therefore required for IBE etching through MTJ stack and BE layers. On the other hand, MTJ components are sensitive to being degraded by the chemical etching ambient, which often degrades TMR (tunnel magneto-resistance). It has been found that the etched surface of MTJ, including the sidewall edge, is damaged in plasma ambient. The damaged depth is estimated to be on the order of several nanometers (nm) from the surface. Tilted angle IBE works to remove the damaged layer. IBE is effective to clean sidewalls.
Another issue is the process sequence and complexity. In some fabrication methods, the main body of MTJ stack and bottom electrode are defined separately using two different photo-masks. Specifically, field MRAM requires the MTJ stack and bottom electrode to be patterned separately. However, separate patterning is not mandatory for STT (Spin Transfer Torque) MRAM. While it is less challenging to fabricate the device from etch point of view, there is a trade-off with process complexity, manufacturing cost, as well as extendibility to high density. In addition to the photo-mask required to pattern MTJ stack, an extra mask is needed to define bottom electrode, which complicates the process flow due to required planarization after each photo processing step, overlay margin tolerance, etc. Also a small cell area cannot be achieved with BE size larger than MTJ size, so this limits extendibility.